A sub-frame is a structural component of a vehicle, such as an automobile or an aircraft that uses a separate structure within a larger body-on-frame or unit body to carry certain components, such as a suspension. The sub-frame is typically bolted or welded to the frame of the vehicle. When bolted, it is sometimes equipped with rubber bushings or springs to dampen vibration.
The principal purposes of using a sub-frame are to spread high chassis loads over a wide area, to isolate vibration and harshness from the rest of the vehicle, and to aid in manufacturing by allowing for a sub-assembly suspension unit to be attached to the vehicle on the assembly line. Sub-assembly suspension units may allow for an easier insertion of an engine, for example, through the bottom of the vehicle during manufacturing, as opposed to through a hood opening, or before a body is married to a frame.
As opposed to an automobile with a full frame and chassis, separate front and rear sub-frames have been used in some modern vehicles to aid in manufacturability and reduce overall cost.
There are generally three basic forms of the sub-frame; an axle type which usually carries the lower control arms and steering rack, a perimeter frame which usually carries the above components but in addition supports the engine, and a perimeter frame which carries the above components but in addition supports the engine, transmission and possibly full suspension, as is sometimes used on front wheel drive cars. A hybrid axle-perimeter type may carry the full suspension along with a steering rack and anti-sway bar.
A sub-frame is usually made of pressed steel panels that are much thicker than bodyshell panels, which are welded or fastened together. Sub-frames are sometimes made of light alloys (aluminum or magnesium) as either single unitary castings or fabricated from a collection of pressed and cast subcomponents. The use of hydroformed steel tubes has also been used.